This invention is related to the preparation of a novel scale inhibiting random copolymer useful in the treatment of water in recirculating cooling water systems. More particularly, it is directed to a method for making a terpolymer of acrylic acid or methacrylic acid (hereafter referred to as "(meth)acrylic" acid to denote either acid), and certain esters of the acids which esters are derived by reaction with a lower alkylene oxide under controlled conditions. The terpolymer is substantially the only constituent of an essentially non-crosslinked random copolymer which inhibits the formation and deposition of scale-forming inorganic salts such as calcium phosphate, calcium carbonate and calcium sulfate in open-loop "process water" including cooling water systems, and boiler feed water systems. Such systems are prone to scale-forming precipitation of calcium, magnesium and iron salts, particularly calcuim and magnesium phosphates, under presently preferred alkaline conditions for operating process water systems in industrial applications.
Much interest has been generated by the teachings of U.S. Pat. Nos. 3,663,448; 4,029,577; 4,324,664; 4,324,684; and 4,326,980 directed to the efficacy of various acrylic copolymers in water treatment, the relevant disclosures of which are incorporated by reference thereto as if fully set forth herein. For example, the '577 patent to Godlewski et al discloses that a copolymer of (meth)acrylic acid ("(M)AA" for brevity) or salt thereof, and, a hydroxylated lower alkyl acrylate ("HAA") is highly effective in controlling the formation and deposition of scale and/or suspended solid matter which would otherwise occur in aqueous media containing scale-imparting ions and dispersed particles. The ratio of (M)AA:HAA may be as high as 34:1 and as low as 1:4.
In U.S. Pat. No. 3,663,448 (Ralston), the formation of solid scale-forming salts in aqueous solution is inhibited by adding to the solution small amounts of certain amino phosphonate compounds, together with a water soluble polymer having a molecular weight from about 500 to about 12,000 selected from the group consisting of polyacrylic acid, copolymers of acrylic acid and up to 50% acrylamide and polyacrylamide in which at least 50% of the amide groups are hydrolyzed.
U.S. Pat. No. 4,209,398 (Ii et al.) discloses yet another water treating process wherein a polymer having a structural unit derived from a monomer having an ethylenically unsaturated bond and having one or more COOH radicals, is combined with inorganic phosphates, phosphonic acids, organic phosphonic acid esters, or polyvalent metal salts, to prevent scale formation and corrosion.
The '664 patent to Snyder teaches that the effectiveness of the '577 water-treating composition is enhanced by the addition of a water soluble ester of an aliphatic sulphodicarboxylic acid. The '684 patent to Geiger et al. teaches that a copolymer of the '577 patent may be combined with a water-soluble zinc compound and a water-soluble chromate compound to enhance the corrosion-inhibiting effectiveness of the combination. The '980 patent to Snyder discloses a composition comprising an acrylic acid/lower alkyl hydroxylated acrylate copolymer which is administered to a water system in combination with an alkyl phenoxy polyethoxyethanol compound.
Thus, having found the effectiveness of a copolymer of acrylic acid and an ester of the acid, a great deal of effort has been expended to find increasingly more effective water treatment compositions which retain the effective acid-ester copolymer configuration. This effort continues apace to produce such compositions effectively and economically.
The terpolymer of this invention is particularly useful in cooling water systems including cooling towers, such as referred to in the foregoing prior art patents, in which systems the term "scale" applies to deposits which result from crystallization or precipitation of salts from solution. Scale formation is influenced by the temperature of the water in a particular location, the concentration of the inorganic salts dissolved in that water, the pH of the water, and other factors. It is this scale formation and deposition which is sought to be inhibited.
The current preference for treating cooling water is with a high pH and/or non-chromate corrosion inhibition program which includes phosphates and other salts which lead to the formation of calcium phosphate and other calcium salt deposits. This is equally true of boiler water systems as detailed in the Betz Handbook of Industrial Water Conditioning, 8th Edition, 1980, published by Betz Laboratories, Inc.
It is known that poly[(meth)acrylic acid] and their salts have been treated with alkylene oxides to produce polymeric esters with such catalysts as pyridine or NaOH and the 2-hydroxyalkyl ester has sites for the further reaction of alkylene groups resulting in the formation of grafted polyoxyethylene sidechains on a backbone of poly[(meth)acrylic acid]. (See "Water-Soluble Resins" by Davidson, R. L. and Sittig, Marshall, 2d Edition, pg 165, Reinhold Book Corporation (1968). This confirms the teaching in U.S. Pat. No. 3,116,270 to Pennino that some condensation of alkylene oxide occurs onto the hydroxy ester groups formed on the polymer chain during reaction although the exact structure of the products was not known. However, alkoxylation resulting in polyalkoxy groups on a polymeric backbone is quite different from alkoxylation of a monomer because of the presence of the unsaturation of the monomer. As indicated in U.S. Pat. No. 4,246,370 to Lewis et al., a base catalyzed reaction with an alkylene oxide generating an alkoxide ion would be expected to result in polymerization.
The '270 Pennino patent teaches a cross-linked copolymer formed by hydroxyalkylation ("alkoxylation") of a pre-prepared interpolymer of (i) from 25 to 75% by weight of an alpha, beta-olefinically unsaturated monocarboxylic acid and (ii) from 75 to 25% by weight of an ester of an alpha, beta-olefinically unsaturated monocarboxylic acid copolymerizable with (i). The copolymer is thermosetting only if less than a molar equivalent of alkylene oxide is used relative to each mole of acid in the copolymer. This is necessary to leave carboxyl groups of one chain to crosslink with pendant hydroxyl groups of another chain. Further, his alkoxylated interpolymer has different types of ester substituents on each of the monomer units, and the Pennino process does not suggest that his esterification is substantially affected by either the type or the amount of basic catalyst used, as is the case with the catalyst-sensitive process of my invention.
There is no question that the hydroxyalkyl acrylate can be prepared by the addition reaction between the acrylic acid or its derivatives or water soluble salts, and the oxide of the alkyl derivative desired. For example, the most preferred monomer of the present invention is the propyl derivative. Accordingly, to obtain the hydroxylated monomer ("HAA.sub.m "), acrylic acid ("AA") is reacted with propylene oxide (PO) to provide the hydroxypropylacrylate monomer ("HPA.sub.m ") constituent of the prior art copolymer, but they failed to produce the hydroxypropyleneoxy ester ("HAA.sub.p "), or if they did accidentally produce the HAA.sub.p, they failed to recognize that they had done so. Under such circumstances it will be appreciated that there could not have been any thought of making the terpolymer of my invention.
Though, given the problem of making the terpolymer of this invention, it is theoretically clear that it can be made, there is no reason for wanting to make such a terpolymer, and no suggestion that if made by the essential manipulative steps of my invention, it would be water-soluble or have a beneficial scale-inhibiting function in recirculating water systems.
Clearly, where a thermosetting water-insoluble polymer with random hydroxyalkyl(meth)acrylate and alkyl(meth)acrylate or other ester units different from the hydroxyalkyl ester units, and, a residual amount of from 5 to 30% free acid is desired, the Pennino process will be used. However, it is not clear whether the "free acid" refers to carboxylic molecules of unreacted monomer, or to COOH groups on reacted monomer units within the polymer. In either case, there is no suggestion that the polymer be modified in any way to be water-soluble, or to contain monomer units having COOH groups in an amount greater than 30% by weight of the polymer. Apparently, because the epoxide constitutes a substantial portion of the polymer and is essential for its cross-linking, the 5-30% free acid provides the desired polymer. Therefore, where a random water-soluble polymer of HAA.sub.m and HAA.sub.p with (M)AA monomer units is desired, it is the process of my invention which will be used.
Of course alkoxylation of (meth)acrylic acid is well known, having been taught in numerous patent references such as U.S. Pat. Nos. 2,484,487; 2,819,296; 3,059,024; and 3,150,167; inter alia, but none was concerned with the subsequent use of the ester formed. Moreover, each used an excess of alkylene oxide in the reaction, whatever the catalyst used, to ensure that all the acid was hydroxyalkylated (or "alkoxylated"), and then, any residual unreacted alkylene oxide was removed to stabilize the esters and minimize the formation of diester during storage (e.g. see U.S. Pat. No. 3,059,024, col 3, lines 10-13).
In addition to Pennino, supra, other patent references which teach carboxyl containing polymers include U.S. Pat. Nos. 2,530,983; 2,607,761; and, 2,908,663, inter alia, but none addresses itself to preparing the copolymer in the presence of an esterification catalyst which is normally reactive with the free radical initiator. Further, one skilled in the art will recognize that the presence of a very small quantity of unreacted alkylene oxide is difficult to avoid in an esterification. If this alkylene oxide is allowed to remain during polymerization, undesirable byproducts would be likely to result and exacerbate the effects of the presence of a hydroxyalkylation(esterification)catalyst. Therefore, the choice of esterification catalyst, though such catalysts are known in the art, is of critical importance to the manipulative steps of my process.
The choice of esterification catalyst found so effective in my invention is particularly unique because it has long been known that amine catalysts such as pyridine, trimethylbenzyl ammonium chloride, choline, triethyl amine and n-methyl morpholine suffer from several serious disadvantages. For one thing, the reaction using amine catalysts goes slowly and high temperatures are necessary to push the reaction to completion with the result that large amounts of acrylic or methacrylic polymer are built up, as stated in the aforementioned '167 patent to Wright et al. Yet it is essential for the purposes of my invention, that particular amine catalysts be used, and it is fortuitous that these catalysts are free from transition metals, so that my water-treatment composition can be put to general use in process water systems.
From a production point of view, it is undesirable to pre-prepare the alkoxylated acrylate monomer, then purify it before polymerizing it. To avoid purification requires purchasing and handling a monoalkoxylated ester, a polyalkyleneoxylated ester and the (meth)acrylic acid. Moreover the hydroxyalkyl (meth)acrylates are quite toxic and require special handling (see Dow Chemical Co., Material Safety Data Sheet, June 11, 1978). Therefore, aside from the cost of the hydroxyalkyl ester, there is a compelling reason, now recognized because of the commercial importance of the desired hydroxyalkylated acid copolymers, to produce the hydroxyalkyl ester in situ, to avoid purchasing it and to avoid handling the material.
For another thing, one seeking to polymerize particular (meth)acrylate esters with (meth)acrylic acid would routinely be duly careful to purify the monomers prior to purification, thereafter making sure they are introduced in the correct proportions to give the desired polymer. It is simply happenstance that the specific catalysts which are effective in producing the desired mix of HAA.sub.m and HAA.sub.p do not adversely affect the polymerization.